I don't know if this is a repost or whatever but i thought this would be pretty useful. Basically you select how big the parcel size is and roughly how much it weighs, as well as the dimensions, and specify where you are sending it from and where it is going, and it will work out the cost for you.

I tried to send a crankshaft to Perth (25 kg) Aust Post rejected it...and even if it had have made the max weight of 20 kg it would have cost $50....greyhound did it for $38....another company quoted me $70.